Telephones typically either do not include any provision for adjusting the volume of the conversation presented to the receiver portion of the telephone handset, or do not have the ability to adjust the volume to a sufficiently high level as required by certain users such as the hearing impaired. Accordingly, where there is a poor connection or the user is hearing impaired, the volume output by the receiver may be undesirably low. The user has little recourse other than asking the other party to speak more loudly. Therefore, it is desirable that the user have available a line amplifier that is readily attached to a standard telephone and adjustable to the user's desired volume level.
Line amplifiers are attached to a telephone between the base part of the phone and the handset and provide receiver gain. Below a certain gain level applied to the receiver, the line amplifier acts merely as a “pass through” for the microphone signal being transmitted unmodified from the handset to the telephone base. When acting as a pass through, it is not necessary to know the polarity of the microphone transmit wires when the line amplifier is inserted or necessary to modify the signal level of the microphone signal.
However, the use of line amplifiers to provide more than a certain level of gain on the receive signal to be output to the telephone user results in acoustic instability between the telephone microphone and receiver. The level below which the circuit is considered to be unconditionally stable is referred to herein as the threshold stability level herein. Above the threshold stability level, acoustic instability may result. For example, acoustic instability may result for amplification above 25 dB to 30 dB in gain. The level at which acoustic instability may occur may vary depending on the particular telephone design. Acoustic instability results from coupling of the receiver output to the microphone input and may result in a screeching, wailing, or other undesirable noise artifact.
In order to provide gain above the threshold stability level without producing acoustic instability, some form of duplexing (attenuation) on the microphone transmit wires is required. In the prior art, this required the need to “break” the microphone wires in the line amplifier to insert polarity sensitive line amplifier circuitry and process the microphone signal received from the handset before it is passed to the telephone base. When the microphone wires are broken, the line amplifier does not merely act as a pass through for the microphone signal.
The electrical connection between the handset and base unit typically includes four wires, two of which are connected to the receiver portion and two of which are connected to the microphone portion. In order for the line amplifier to process or attenuate the signal on the microphone wires, the polarity of each wire of the microphone wire pair (i.e., ground or signal) must be matched with the line amplifier circuit.
Because neither the microphone wiring polarity or the output level of the microphone in the handset can be known in advance by the line amplifier manufacturer, various polarity and amplitude settings need to be provided by a compatibility switch or switches on the line amplifier unit. Previous solutions that interrupt the microphone wires for processing, have utilized some type of complicated switching matrix to account for the different polarity possibilities. In addition, different microphone sensitivities may be used by different phone manufacturers, so that the line amplifier must provide a microphone gain adjustment to prevent the output from the line amplifier being too low or too high on the microphone pair. While these solutions work, they require complicated circuitry and may be difficult for a user to operate. This introduces complexity for the user in setting the unit up to work properly with their phone.
As a result, for these and other reasons, there is a need for improved methods and apparatuses for line amplifier circuits.